1. Field of the Invention
The present invention relates to an interchangeable lens attachable to an image pickup apparatus capable of performing so-called contrast AF.
2. Description of the Related Art
Image pickup apparatuses such as digital still cameras and video cameras acquire or produce, from a video signal obtained through photoelectric conversion of an object image by an image sensor, an focus evaluation value signal (focus information) that shows a contrast state of video (imaging contrast). The image pickup apparatuses move a focus lens to an in-focus position where the focus evaluation value signal becomes maximum to perform a contrast detection method autofocus (contrast AF). The contrast AF is also called TVAF.
For the contrast AF, a relationship between the focus evaluation value signal and a position of the focus lens for acquisition thereof is important. Particularly, in a lens-interchangeable image pickup apparatus to which an interchangeable lens is detachably attachable, since the image pickup apparatus produces the focus evaluation value signal and the interchangeable lens drives the focus lens, it is necessary to appropriately manage timings for acquisition of the focus evaluation value signal and timings for drive of the focus lens.
Japanese Patent Laid-Open No. 2009-258718 discloses a method of setting drive timings of a focus lens in a lens-interchangeable image pickup system performing contrast AF. Moreover, Japanese Patent Laid-Open No. 2011-257450 discloses a method for improving focusing accuracy in which an interchangeable lens performs an in-focus position calculation using focus lens positions detected according to a synchronization signal from an image pickup apparatus performing contrast AF.
If the lens-interchangeable image pickup apparatus is capable of managing the drive timing of the focus lens without delay with respect to the acquisition timing of the focus evaluation value signal, it is possible to always acquire the focus evaluation value signal at appropriate focus lens positions and there is no problem. However, it is difficult for the image pickup apparatus to perform communication with the interchangeable lens to send thereto an instruction for causing the interchange lens to start drive of the focus lens and an instruction for causing the interchangeable lens to acquire a reaching position of the focus lens without timing delay while performing other processes.
To solve the problem, the image pickup apparatus may perform prediction calculation of the focus lens position. However, interchangeable lenses of mutually different types perform mutually different speed controls (acceleration and deceleration controls) for an actuator such as a motor. Therefore, it is not easy for the image pickup apparatus to accurately perform the prediction calculation of the focus lens position without delay with respect to actual drive of the focus lens.